1. Field of the Invention
The present invention relates to an optical beam scanning apparatus in which a plurality of optical beams modulated in accordance with image signals are cyclically deflected by deflecting means and are then scanned to record images on a photosensitive material.
2. Description of the Prior Art
A short drawing time is desired in an optical beam scanning apparatus such as a laser plotter for recording an image as a black-and-white image on a photosensitive material and a scanner for producing a printing plate by recording a gradation image consists of halftone dots on a photosensitive material. For this reason, an optical beam scanning apparatus which scans a plurality of laser beams is commonly used.
In such an optical beam scanning apparatus, one thing is important, that is, a varying beam-to-beam distance. If drawing density changes, a beam-to-beam distance must change accordingly. To fulfill this demand, beam-to-beam distance adjusters have been used. For example, a beam-to-beam distance adjuster disclosed by the inventor of the present invention in the patent application of Japanese Patent Application No. 1-89460 (heretofore referred to as "the precedent application").
FIG. 1 is an explanatory diagram illustrating principles of beam-to-beam distance adjustment according to which a beam-to-beam distance adjuster is constructed. In FIG. 1, a beam-to-beam distance adjuster comprises a beam director 301 which is the same in structure as a beam splitter. With its half mirror surface 302, the beam director 301 allows a first beam B.sub.1 to partially pass through the same to become a beam B.sub.a while reflecting a part of a second beam B.sub.2 to make it a reflected beam B.sub.b. The beams B.sub.a and B.sub.b, directed by the beam director 301 which is rotationally displaced according to an adjustment rule described later, intersect in a rear side at a predetermined point P.sub.C at an angle of .theta.. At the intersection P.sub.C, an acousto-optic deflector 213 is disposed to cyclically deflect the beams B.sub.a and B.sub.b. The beams B.sub.a and B.sub.b enter the AOD 213 where they would be deflected, and are then directed to a scanning lens 216 where they would be converted to become parallel beams. The distance between the intersection P.sub.C and the scanning lens 216 is equal to the focal length f of the scanning lens 216.
As in FIG. 1, the following relationship holds between a distance l.sub.O between the parallel beams B.sub.a and B.sub.b, the intersection angle .theta. and the focal length f of the scanning lens 216: EQU l.sub.O .perspectiveto..theta..multidot.f (1)
Hence, the distance l.sub.O between the beams B.sub.a and B.sub.b can be properly changed by changing the intersection angle .theta.. Still, one thing must not be forgotten in executing such beam-to-beam distance adjustment by changing the intersection angle .theta.. The beams B.sub.a and B.sub.b must intersect at the intersection P.sub.C so that they never fail to enter the AOD 213 even if the intersection angle .theta. is changed. This is partly because the AOD 213 has an entrance side aperture of a rather small diameter and partly because the larger the diameter of the beams B.sub.a and B.sub.b, the better focused the beams B.sub.a and B.sub.b would be on a photosensitive material 1. Simply increasing the intersection angle .theta. would lead to an intersection P.sub.C gradually shifting toward the beam director 301 and would finally end up in a situation that the beam B.sub.b does not enter the AOD 213, because the diameter of the beams B.sub.a and B.sub.b is customarily set equal to the diameter of the entrance side aperture of the AOD 213.
The below will describe the adjustment rule for changing the intersection angle .theta. while always satisfying the condition that the beams B.sub.a and B.sub.b intersect at the intersection P.sub.C. As mentioned before, the beam-to-beam distance adjuster changes the intersection angle .theta. by rotationally displacing the beam director 301 around a predetermined rotation center C.sub.R. The rotation center C.sub.R of the beam director 301 is set at a position of the element 301 of when the half mirror surface 302 is placed at a position 302a shown in FIG. 2. A distance A between the reference position of the beam director 301 and the rotation center C.sub.R depends on an optical length a which is a distance between the reference position and the intersection P.sub.C. Hence, the distance A must be set relative to the optical length a.
FIG. 2 is a conceptual diagram to find how to set the distance A. It is assumed that the beams B.sub.a and B.sub.b intersect at the intersection P.sub.C at angle of .theta., and that the beam director 301 (not shown in FIG. 2) is rotated for an angle of .beta. about the rotation center C.sub.R. In FIG. 2, the lines and the symbols stand for:
P.sub.R . . . a reflection point on the half mirror surface 302 at which the beam B.sub.2 is reflected; PA0 F.sub.H . . . a line drawn through the point P.sub.R to be parallel to the optical path of the beam B.sub.1 ; PA0 F.sub.N . . . a normal line to the half mirror surface 302; PA0 F.sub.45 . . . a line inclined at an angle of .pi./4 to th incident direction of the beam B.sub.2 ; PA0 .alpha. . . . an angle between the direction of the beam B.sub.2 and the line F.sub.N ; and PA0 h . . . a height of the point P.sub.R taken from the optical path of the beam B.sub.1. PA0 Length of the arm member 501 . . . 30 mm PA0 Length of the arm member 502 . . . 40 mm PA0 Length of the arm member 503 . . . 30 mm
In the case as above, an intersection angle between the line F.sub.N and the line F.sub.H is .alpha.-.theta. and an intersection angle between the line F.sub.H and the beam B.sub.2 and is .pi./2. Hence, EQU (.alpha.-.theta.)+.alpha.=.pi./2 (2)
Since an angle between the line F.sub.45 and the line F.sub.H is .pi./4, EQU (.alpha.-.theta.)+.beta.=.pi./4 (3)
Now, eliminating .alpha. from Eqs. 2 and 3 yields EQU .theta.=2.multidot..beta. (4)
At the same time, EQU h=a.multidot.tan .theta. (5)
and if .theta. is enough small, h is approximately EQU h=a.multidot..theta. (6)
Hence, obtained from Eqs. 4 and 6 is EQU h=2a.multidot..beta. (7)
In addition, the following holds: EQU tan .beta.=h/A (8)
Now, combining Eqs. 4 and 8 gives ##EQU1## On the other hand, Eq. 5 can be rewritten as EQU cos .theta.=cos [tan.sup.-1 (h/a)] (10)
and therefore, substituting Eq. 10 in Eq. 9 gives Eq. 11 which shows the distance A changes according to a change in the distance a. EQU A=(a.sup.2 +h.sup.2).sup.1/2 +a (11)
In Eq. 11, a and h take the following values for example: EQU a=300 mm (12) EQU h=0.04 mm (13)
Thus, h usually takes a pretty small value compared to a. Due to this, Eq. 11 can be regarded as Eq. 14. EQU A.perspectiveto.2a (14)
In short, the rule for adjusting a beam-to-beam distance is summarized as follows: The beam director 301 is moved in a parallel direction while rotationally displaced about a point so as to satisfy Eq. 7, the point being twice far the optical path length a between the reference position 302a and the AOD 213 from the reference position 302a.
FIG. 3 is a diagram schematically showing a mechanism of a beam-to-beam distance adjuster disclosed in the precedent application (No. 1-89460). In FIG. 3, the beam-to-beam distance adjuster includes a four-node link mechanism 500 consists of arm members 501, 502 and 503. When, the four-node link mechanism 500 operates, this structure makes the center member 502 (that is, the beam director 301) rotate about a point C.sub.A. The point C.sub.A functions as a center point of rotation for a moment. This implies that if the link mechanism 500 is designed so that a distance between the beam director 301 and the point C.sub.A takes a value of 2a, the beam director 301 can be rotationally displaced in such a manner that the point C.sub.A serves as the rotation center C.sub.R of FIG. 1 while approximately satisfying Eq. 7.
Attention is directed here to that the beam-to-beam distance adjuster of FIG. 3 only approximately satisfies Eq. 7. Hence, as the angle .theta. grows, a deviation from Eq. 7 increases to such a degree that it cannot be ignored anymore. Let the distance a be 250 mm and the link mechanism 500 be dimensioned as below, for example, in order to show this.
Distance between the right end of the member 501 and the left end of the member 503 . . . 37.6 mm
Now, if the angle .theta. is 1.1 mrad, the beams B.sub.a and B.sub.b will intersect at a point nearly 1 mm far from the point P.sub.C. If the angle .theta. grows to 10 mrad, the beams B.sub.a and B.sub.b will intersect at a point as far as 10 mm from the point P.sub.C and fail to enter the AOD 213.
Aside from the above, some beam-to-beam distance adjusters comprise a beam expander with magnification of M.sub.0 disposed between the beam director 301 and the scanning lens 216 in order to enlarge a diameter of beams which enter the AOD 213, which would eventually increase the number of resolution points through scanning. When such is adopted, although the diameter of the beams is expanded by M.sub.0 times at the beam expander, an angle between the beams becomes 1/M.sub.0 larger. Assume that two beams from the beam director 301 are at an angle of .gamma. to one another. The beams passed through the beam expander would have a relative angle of .gamma./M.sub.0. Hence, two beams from the beam director 301 must be at an angle of M.sub.0 .multidot..theta. to one another in order to obtain two beams entering the scanning lens 216 at an angle of .theta. to one another. This means that the beam director 301 must be considerably rotated. However, such would lead to an unwanted result that beams which are supposed to intersect at the point P.sub.C meet at a point far from the point P.sub.C.
An apparatus shown in FIG. 4, for instance, solves the problems as above. In the apparatus, the beam director 301 is fixed to a stage 600 which is supported by piezoelectric elements 601 and 602 through notched links. In this structure, the beam director 301 is rotationally displaced in the direction of .theta..sub.a while satisfying Eq. 7 by differently extending the piezoelectric elements 601 and 602.
The beam-to-beam distance adjuster of FIG. 4, however, has a problem. Since two actuators (piezoelectric elements) must be employed, the apparatus needs complicated control and would be costly.